1. Field of the Invention
The present invention relates to an apparatus and method for monitoring an optical signal in a high-speed large-capacity optical communication system.
2. Description of the Prior Art
With the increase in demand for communications, optical communication systems are increasing rapidly in capacity. Under circumstances where the transmission capacity per optical fiber is increased to the order of terabits by using wavelength division multiplexing (WDM), there is a need to increase the signal speed per wavelength in order to further increase the capacity. The development of techniques for high-speed signal transmission at a rate of 40 Gbits/s or higher is being pursued. Monitoring processing on a degraded waveform is conceivable as a means for realizing long-distance transmission of a high-rate signal of 40 Gbits/s or higher, which can be easily affected by dispersion in an optical fiber and nonlinearity of the optical fiber, and the waveform of which can degrade easily in comparison with a low-rate signal. In particular, chromatic dispersion or polarization mode dispersion in a transmission path fiber changes with time due to a change in temperature of the fiber for example. Under the influence of such changing dispersion, the possibility of the Q-value being reduced is high. It is, therefore, important to realize real-time monitoring of chromatic dispersion and polarization mode dispersion and control of compensation for such dispersions.
Several methods for directly measuring the value of chromatic dispersion or polarization mode dispersion have been proposed. For example, a method of estimating a dispersion value from the intensity of a clock frequency component which is contained in an optical signal has been proposed (see patent document 1).
It is thought that limitation of the reduction in Q-value by monitoring a signal waveform in a real-time manner and by controlling a compensator for compensation for chromatic dispersion, polarization mode dispersion or the like is also effective.
A typical example of means for monitoring the waveforms of optical signals is a sampling oscilloscope which is capable of observing a high-speed signal waveform at a low sampling frequency, and which is ordinarily used as a measuring apparatus for signal waveform evaluation of high-speed electric and optical devices, etc.
Further, as a means for extracting waveform information in a simpler manner in comparison with the sampling oscilloscope, an amplitude histogram method has been proposed. For example, waveform degradation due to noise, crosstalk, or chromatic dispersion is observed from a light intensity distribution extracted by asynchronous sampling (see non-patent document 1). Extraction of a clock signal from data is not required for this method since data is sampled in an asynchronous manner. Therefore, this method has the advantages of using a simplified apparatus and enabling measurement theoretically independent of the bit rate of the signal.
[Patent Document 1]
Japanese Patent Laid-Open No. 11-68657
[Non-Patent Document 1]
Electronics Letters, 1999, 35, pp. 403 and 404
The above-described techniques, however, have problems described below.
In the case of the method of estimating a dispersion value from the intensity of a clock frequency component contained in an optical signal, the state of waveform degradation cannot be determined with accuracy and there is a problem in terms of accuracy of control, because the change in clock output intensity with respect to a change in dispersion is small. Moreover, a special clock extraction circuit is required for monitoring of the clock intensity and a phase-locked loop (PLL) circuit ordinarily used as a clock extraction circuit cannot be used.
In optical waveform monitoring on an optical signal using a sampling oscilloscope, the scale of an apparatus for monitoring a signal in an actual communication system is so large that an application of this monitoring to an optical receiver is not practicable. The amount of data obtained by a sampling oscilloscope is considerably large and the cost of signal processing for extracting suitable parameters is high. A comparatively long time is required to depict a waveform for a reason in terms of principle, which is a problem with use of this technique for real-time control. In the case of observation of an actual high-speed optical waveform, it is necessary that each of optical and electrical devices used in the sampling oscilloscope have a sufficiently wide frequency band, and the cost of preparation of high-speed devices is considerable. If some of the optical and electrical devices do not have a sufficiently wide frequency band, the observed waveform may become different from the desired form, depending on the optical and electrical devices, and it is difficult to correctly perform analysis or to extract correct information.
In the case of the amplitude histogram method, the same performance as that of a sampling oscilloscope is required. For example, it is necessary that a device for receiving an optical signal have a characteristic of a sufficiently wide band, and that the width of a sampling gate be sufficiently narrow, although sampling is performed in an asynchronous manner. Therefore, an excessively large cost is required for application of this method to an actual system. There are also the same problems as those with a sampling oscilloscope, e.g., the problem that the cost for signal processing is large and the time required for information extraction is long. Further, it is difficult to observe variation in the time direction, e.g., spreading of signal pulses by using this means.